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Set the oven to Gas Mark 7 or 220C. To make the choux pastry: Place butter in a saucepan and pour in 150ml ( pint) water. Place pan over a medium heat until the butter melts. Increase heat and bring mixture to the boil. Remove pan from heat, quickly tip in the flour and beat mixture until it binds together and comes away from the sides of the pan to form a ball. Leave the mixture to cool for at least 10 minutes, then beat in the eggs, one at a time. Fill piping bag fitted with the plain piping tube, with the choux mixture. Pipe out lines of the mixture, about 7-8cm (2 - 3in) long, on the lined baking sheets. Sprinkle some cold water over them, then bake in the centre of oven for 15-20 minutes, swapping the position of the trays if the ones on the top start to brown quicker than the lower ones. Remove the trays with the clairs from the oven. Pierce each clair with a skewer to allow the steam to escape, then return to the oven for a further 5-7 minutes or until they are crisp. Remove the trays from oven and transfer the clairs to a wire rack. Leave them to cool completely. Cut one side of each clair and scoop out any soft mixture in the centres using a teaspoon. To make the filling: Break the chocolate into pieces and place it in a bowl. Pour cream into a pan and bring to the boil, then pour it over the chocolate. Stir until the chocolate dissolves, and add vanilla extract to taste. Allow the mixture to cool, then chill until it?s thick. Whisk the chocolate filling until light and fluffy and holding its shape, then use to fill the piping bag, fitted with the star piping tube. Pipe some of the chocolate mixture into each clair, then press the clairs lightly to close them. To make the topping: Place the chocolate and butter in a bowl and melt, either in a microwave, or over a pan of hot water. Dip the top of each clairs into the mixture, taking care not to get any of the topping in the filling. This may be easier to do if you brush it on with a pastry brush. Leave the topping to set before serving.